This invention generally relates to the widely used continuous or semicontinuous D.C. (direct chill) casting of metal products, particularly light metal products such as aluminum. In this casting process, molten metal is introduced into the feed end of an open-ended, tubular mold and, as this metal stream passes through the mold bore, it is cooled and thus solidified at least in part. When the metal emerges from the discharge end of the mold, it is sprayed with coolant to complete metal solidification.
Molten metal introduced into the feed end of the mold rapidly forms a thin layer of solidified metal immediately adjacent the chill surfaces of the mold bore. As the solidification continues, the metal stream contracts and pulls away from the mold wall, and, once contact is lost with the chill surfaces of the mold bore, relatively little heat removal is effected through the mold walls. Thereafter, essentially all solidification is effected by the application of coolant onto the metal as it emerges from the discharge end of the mold.
The solidification of metal in the D.C. casting process usually creates extremely large thermal gradients between the surface and the center of the solidified metal. These large thermal gradients can develop internal stresses which become so large that cracks and splits develop in the solidified metal during or shortly after the casting thereof. The high strength, highly alloyed aluminum alloys, such as the 7XXX (Al-Zn and Al-Zn-Mg) and the 2XXX (Al-Cu) alloys (Aluminum Association alloy designations), are particularly susceptible to such cracking. Cracking severely reduces the metal recovery rates and thus increases costs because the cracked ingots must either be scrapped or severely cropped.
It has long been recognized, for example, by Zeigler in U.S. Pat. Nos. 2,708,297 and 2,705,353 and Elliott et al in U.S. Pat. No. 3,653,425, that cracking of highly alloyed, D.C. cast ingot or billet could be minimized by removing the coolant from the solidified metal surfaces soon after it is applied to the metal emerging from the discharge end of the mold. By quickly removing the coolant, the high temperature metal at the center of the ingot or billet reheats the cooler metal at the surface resulting in stress relief which prevents or minimizes cracking.
Many types of coolant removing devices such as rubber blades, air knives, rotating brushes or rolls, and the like, have either been suggested or actually employed over the years in an attempt to prevent ingot and billet cracking. However, except for the rubber blade, most of these devices have not been used commercially to any significant extent because they were not practical in the industrial environment for which they were designed. Although frequently used, rubber blades would often be torn or eroded by the rough surface of the butt of the ingot or billet which is formed initially during casting and thereby allow coolant to leak onto the wiped metal surfaces and result in ingot or billet cracks. Additionally, rubber blades would not remove coolant from the metal surface in a uniform manner particularly at corners, where coolant leakage usually occurs. The use of rotating devices such as brushes and rollers presented problems similar to the blade wipes and additionally such devices could not be used on metal having a circular cross section. Air knives were effective in removing coolant but they required excessive volumes of air which rendered them impractical.
Over the years the casting art has progressed to the point that by slightly modifying alloy compositions, improving casting procedures and improving casting mold designs, highly alloyed light metals could be cast in a regular manner. However, even with this progress metal recovery was, and still is, frequently quite low. For example, with highly alloyed, high strength, aluminum alloys such as 7050 aluminum alloy, a metal recovery of 50% is sometimes considered successful, and most of such metal losses are due to cracked ingots.
It is against this background that the present invention was developed.